1. Technical Field
The present invention relates to a fuel cell system and, more particularly, to a fuel cell system having a structure to improve the fuel efficiency of the entire system.
2. Related Art
A fuel cell is a system for producing electric power. In a fuel cell, chemical reaction energy between oxygen and hydrogen contained in hydrocarbon-group materials, such as methanol, ethanol and natural gas, is directly converted into electric energy.
Depending on the type of electrolyte used in the fuel cell, the fuel cell is classified into different types: phosphate fuel cell, molten carbonate fuel cell, solid oxide fuel cell, and polymer electrolyte or alkali fuel cell. Although each of these different types of fuel cells operates using the same principles, the types differ in the type of fuel, catalyst, and electrolyte used, as well as in drive temperature.
A polymer electrolyte membrane fuel cell (PEMFC) has been developed recently. Compared to other fuel cells, the PEMFC has excellent output characteristics, a low operating temperature, and fast starting and response characteristics. The PEMFC may be used as a power source for vehicles, in homes and in buildings, and in electronic devices. The PEMFC, therefore, has a wide range of applications.
The basic components of the PEMFC are a stack, reformer, fuel tank, and fuel pump. The stack forms a main body of the fuel cell. The fuel pump supplies fuel in the fuel tank to the reformer. The reformer reforms the fuel to create hydrogen gas, and supplies the hydrogen gas to the stack. Accordingly, the PEMFC sends the fuel in the fuel tank to the reformer by operation of the fuel pump. The fuel is reformed in the reformer to generate hydrogen gas, and the hydrogen gas is chemically reacted with oxygen in the stack to thereby generate electric energy.
In the above fuel cell system, the stack is structured so as to include a few to a few tens of unit cells realized with a membrane electrode assembly (MEA), with separators provided on both sides thereof. In the MEA, an anode electrode and a cathode electrode are provided in opposition to one another with an electrolyte layer interposed therebetween. Further, the separator is realized using the well-known bipolar plate, and acts to separate each of the MEAs. The separator also functions to provide a pathway through which hydrogen gas and oxygen, which are required for fuel cell reaction, are supplied to the anode electrode and cathode electrode of the MEA. In addition, the separator functions as a conductor for connecting the anode electrode and cathode electrode of each MEA in series. Accordingly, hydrogen gas is supplied to the anode electrode and oxygen is supplied to the cathode electrode via the separator. An oxidation reaction of the hydrogen gas occurs in the anode electrode, and a reduction reaction of the oxygen occurs in the cathode electrode. Electricity is generated by the movement of electrons occurring during this process. Heat and moisture are also generated.
In the fuel cell system described above, the stack must be continuously maintained at a suitable temperature to ensure stability of the electrolyte layer and prevent a reduction in performance. To achieve this, the conventional fuel cell system typically includes an air-cooled cooling device to cool the stack using air that is cooler than the heat radiating from the stack. A water-cooled cooling device, in which cool water is used to reduce the heat of the stack, is also commonly used.
However, a drawback of the conventional fuel cell system utilizing such an air-cooled or water-cooled system is that the air or water heated after cooling the stack is simply discarded. This is a tremendous waste of energy.
In addition, only part of the air supplied to the cathode electrode is reacted, while the rest is exhausted as moisture and high temperature steam generated during the generation of electricity. When water is exhausted from the stack to the atmosphere of a relatively low temperature, the water contacts the atmosphere to thereby generate condensation. Therefore, the water escapes from the outer case of the electronic device, thereby imposing discomfort on the user.
Furthermore, in the conventional fuel cell system, the reformer and the stack are often pre-heated prior to first starting the system. The energy used to perform this pre-heating reduces the overall efficiency of the system.
Finally, the conventional fuel cell system uses a configuration in which hydrogen gas is generated through the reformer by separately heating and vaporizing fuel required for electricity generation of the stack. The overall efficiency of the system is again reduced by the energy used to heat the fuel in the reformer to a required temperature.